1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device, and more particularly to a non-volatile semiconductor memory device which information can be electrically written to and erased from. And the invention relates to a method for fabricating the same.
2. Description of the Prior Art
FIG. 8C shows a cross section of a portion of a prior art non-volatile semiconductor memory device (EEPROM: electrically erasable programmable read only memory) which information can be electrically written to and erased from. Such an EEPROM is disclosed, for example, on page 108 in ISSCC, 1982 by Giora Yavon et al.
Referring to FIGS. 8A to 8C, we will describe a prior art method for the fabrication of a non-volatile semiconductor memory device. First, as shown in FIG. 8A, a source region 102a and a drain region 102b, each formed by a diffusion layer, are formed in a silicon substrate 101, and then a first gate insulating film 103 is formed on the silicon substrate 101. The formation of the diffusion layers is achieved by implantation of impurity ions. Ions of opposite conductivity type to that of the silicon substrate 101 are used as the impurity ions. After that, a photoresist (not shown) is applied over the first gate insulating film 103. The photoresist has a window region that defines the plane pattern of a window 109 to be opened in the first gate insulating film 103.
Using the photoresist as an etching mask, dry etching is performed to etch the first gate insulation film 103, thereby opening the window 109 in the first gate insulating film 103 to expose a surface region 107 of the silicon substrate 101. The window 109 is formed above the drain region 102b. As a result of the dry etching, the surface region 107 of the silicon substrate 101 is over-etched and a recess is formed on the surface region 107. The height of the recess is approximately 5 nm to 10 nm. When wet etching is used instead of the dry etching, formation of a recess resulting from over-etching will not occur. However, when a cleaning process using NH.sub.4 OH or other ammoniacal solution is performed after that, the surface region 107 is etched and a recess is formed on the surface region 107.
Next, as shown in FIG. 8B, a second gate insulating film 110 is formed on the drain region 102b by selectively oxidizing the exposed surface region 107. After that, as shown in FIG. 8C, a floating gate 111 is formed on the first gate insulating film 103, after which a third gate insulating film 113 is formed over the floating gate 111 by oxidizing the surface of the floating gate 111, while a control gate 114 is formed above the floating gate 111, thus completing the fabrication of a non-volatile semiconductor memory device.
FIG. 8D shows a cross section of the second gate insulating film 110 of the non-volatile semiconductor memory device thus fabricated. As shown in FIG. 8D, since a recessed portion is formed on the surface region 107 because of over-etching, the edge portion of the second gate insulating film 110 is bent and has a reduced thickness. FIG. 8E is a plan view showing the layout of the second gate insulating film 110 and the floating gate 111 in the non-volatile semiconductor memory device.
Thus, according to the prior art method, a recess is formed on the surface of the silicon substrate 101 during the processing step of opening the window 109 in the first gate insulating film 103. Since the edge portion of the second gate insulating film 110 is bent because of the thus formed recess, electric fields concentrate in the edge portion of the second gate insulating film 110 during operation, making the second gate insulating film 110 prone to dielectric breakdown. Furthermore, the reduced thickness of the edge portion of the second gate insulating film 110 results in a decreased dielectric strength. Therefore, the prior art non-volatile semiconductor memory device has the disadvantage that when writing and erasing operations are repeated, the second gate insulating film 110 suffers breakdown in a relatively short time, which means that the life time of the memory device is short.